Methylamines are generally synthesized from methanol and ammonia at around 400° C. in the presence of a solid acid catalyst, e.g., silica-alumina. As is well known, use of the silica-alumina catalyst leads to predominant production of trimethylamine according to thermodynamic equilibrium although trimethylamine is least demanded among the three types of methylamines, namely, mono-, di- and tri-methylamines. Since dimethylamine accounts for most of the demand for methylamines, methods have been recently developed to selectively produce dimethylamine, overcoming the thermodynamic equilibrium composition.
Some of these methods include those using zeolites (crystalline aluminosilicate molecular sieves), e.g., Zeolite A (see, for example, Patent Document 1), FU-1 (see, for example, Patent Document 2), ZSM-5 (see, for example, Patent Document 3), ferrierite and erionite (see, for example, Patent Document 4), ZK-5, Rho, chabazite and erionite (see, for example, Patent Document 5) and mordenite (see, for example, Patent Documents 6, 7, 8 and 9).
These methods deal with zeolites small in micropore channel size, and try to improve selectivity for dimethylamine and catalytic activity by subjecting them to ion exchanging, dealumination, doping with a specific element or silylation in order to control micropore channel size or modify acid sites on external surfaces thereof.
Also known is a method for production of monomethylamine or the like at a higher proportion than the thermodynamically equilibrium composition by use of a crystalline silicoaluminophosphate molecular sieve (hereinafter referred to as SAPO in principle) as a non-zeolitic molecular sieve (see, for example, Patent Document 10).
After having extensively studied selective dimethylamine production techniques, the inventors of the present invention have also found that SAPOs modified with silica or other various oxides exhibit higher activity and dimethylamine selectivity than conventional zeolite catalysts, and have already applied for patents (see, for example, Patent Documents 11, 12 and 13).
It is generally considered that crystalline molecular sieves, e.g., zeolites and SAPOs, have a surface consisting of an exposed crystal lattice surface. Therefore, the surface of zeolites and SAPOs has strong acid sites which are characteristic of molecular sieves that contain silicon and aluminum. In order to improve selectivity for dimethylamine production, the surface acid sites are often subjected to chemical vapor deposition using silicon compounds, liquid-phase silylation, modification with a boron- or phosphorus-containing compound, or the like (see, for example, Patent Documents 14, 15, 16, 17 and 18). The techniques already described by the present inventors in the patent publications (see Patent Documents 11, 12 and 13) are also based on these concepts.
However, these conventional processes require lots of steps since they require separation and washing by centrifugation or filtration after synthesis of SAPOs and, in some cases, further modification of calcined catalysts. In particular, silylation needs precise control of moisture content of catalysts to be treated, and uses organic solvents, e.g., ethanol or toluene, thereby causing problems of disposal of waste fluid.
In this way, these conventional processes, in which synthesized SAPOs are further modified to improve activity and selectivity, involve problems to be solved both economically and environmentally. There are demands for development of new methods which can control activity and selectivity during steps for synthesis of SAPOs and thus differently from the conventional processes that require the post-treatment steps.
In order to solve these problems, the present inventors have already found that an 8-membered ring SAPO of chabazite structure having a rectangular parallelepiped or cubic crystal form of 5 μm or less in average grain diameter works as an excellent methylamine production catalyst of high activity and dimethylamine selectivity, and have already applied for patent (see Patent Document 19), in addition to the above-described techniques.
This 8-membered ring SAPO of chabazite structure exhibits excellent activity and selectivity without modification after synthesis. However, the present inventors have further studied it and observed a phenomenon of catalyst performance depending upon some synthesis lots. Therefore, it involves technical problems that must be further improved to obtain an optimum catalyst stably.
The 8-membered ring SAPO of chabazite structure having the above-described particle size and shape is synthesized using organic amines or organic ammonium salts, e.g., diethanolamine or tetraethyl ammonium hydroxide in an amount of 1.5±0.5 times the mole number of the aluminum compounds as Al2O3. On the other hand, the amount of the organic amine or organic ammonium salt incorporated into the 8-membered ring SAPO of chabazite structure just after synthesized but before calcined is an amount required to establish the crystalline structure, namely, only about 0.37 times the mole number as Al2O3. The surplus organic amines or organic ammonium salts, which are not incorporated into the crystalline structure, should be treated by activated sludge or the like. Therefore, synthesis of a pure 8-membered ring SAPO with a reduced amount of organic amines or organic ammonium salts would be a desirable process both economically and environmentally.
Since 8-membered ring SAPOs exhibit relatively high selectivity for dimethylamine in methylamine synthesis, various studies have conventionally been made on the synthesis of methylamines using such SAPOs (see, for example, Non-patent Document 4). As 8-membered ring SAPOs, are known SAPO-14, -17, -18, -33, -34, -35, -39, -42, -43, -44, -47 and -56 (see, for example, Non-patent Documents 5 and 6). Among them, three types, namely, SAPO-34, SAPO-44 and SAPO-47 are known to have the chabazite structure. In particular, the SAPO-34 of chabazite structure has been extensively studied as a catalyst for methylamine synthesis and methanol conversion (see, for example, Patent Documents 10 and 21).
It is known that these 8-membered ring SAPOs can be synthesized by hydrothermally treating a mixture of a silicon compound, aluminum compound, phosphorus compound and water in the presence of a structure directing agent such as tetraethyl ammonium hydroxide, morpholine, cyclohexylamine and diethylethanolamine (see, for example, Patent Document 20). It is not known, however, that these 8-membered ring SAPOs have an amorphous oxide layer on their crystal grain surfaces, and the amorphous oxide layer grows on crystal grain surfaces during the hydrothermal synthesis and has a great effect on yield and selectivity of the methylamine synthesis.
In general, the mixture to be hydrothermally treated to synthesize 8-membered ring SAPOs is represented by the compositional formula (1) shown below, and the structure directing agent R is used in an amount of 1 to 3 times the mole number of Al2O3 in the mixture (a/c=1 to 3):aR, bSiO2, cAl2O3, dP2O5, eH2O  Formula (1)
Survey of the amount of structure directing agents used for synthesis of 8-membered ring SAPOs on various patent documents and literatures reveals that structure directing agents are used in an amount of at least one times the mole number of Al2O3 in the mixture to be hydrothermally treated, in order to produce 8-membered ring SAPOs of chabazite structure with high purity and high degree of crystallinity, and it is mentioned that, when they are used in a smaller amount, there occur impurities, e.g., a SAPO-5 structure which falls under the AFI structure according to the IUPAC structural code specified by the International Zeolite Association (IZA), as well as cristobalite or berlinite of aluminum phosphates.
A literature describes synthesis of SAPO-34 using tetraethyl ammonium hydroxide (TEAOH) as a structure directing agent (see, for example, Non-patent Document 1), and discusses that pure SAPO-34 is produced at a TEAOH/Al2O3 molar ratio of 2 to 3, SAPO-5 is produced at a ratio of 1 to 2, and a high-density phase is produced at a ratio below 1. It is also mentioned that grains having a non-crystalline amorphous structure are produced at a ratio above 3.
A literature describes synthesis of SAPO-34 using morpholine as a structure directing agent (see, for example, Non-patent Document 2), and discusses that a mixture of aluminum phosphate cristobalite and an amorphous compound is produced at a morpholine/Al2O3 molar ratio of 0.5 or less, a mixture of 80% SAPO-34 and 20% cristobalite is obtained at a morpholine/Al2O3 molar ratio of 1.0, and pure SAPO-34 is obtained at a morpholine/Al2O3 molar ratio of 2.0 or more.
In addition, a literature describes synthesis of SAPO-44 using cyclohexylamine as a structure directing agent (see, for example, Non-patent Document 3), and discusses that contamination with SAPO-5 occurs at a cyclohexylamine/Al2O3 molar ratio of 1.9 or less.
The above-cited references are as follows:
Patent Document 1: Japanese Patent Laid-Open No. S56-69846A,
Patent Document 2: Japanese Patent Laid-Open No. S54-148708A,
Patent Document 3: U.S. Pat. No. 4,082,805 specification,
Patent Document 4: Japanese Patent Laid-Open No. S56-113747A,
Patent Document 5: Japanese Patent Laid-Open No. S61-254256A,                Patent Document 6: Japanese Patent Laid-Open No. S56-46846A,        
Patent Document 7: Japanese Patent Laid-Open No. S58-49340A,
Patent Document 8: Japanese Patent Laid-Open No. S59-210050A,
Patent Document 9: Japanese Patent Laid-Open No. A59-227841A,
Patent Document 10: Japanese Patent Laid-Open No. H02-734A,
Patent Document 11: Japanese Patent Laid-Open No. H11-35527A,
Patent Document 12: Japanese Patent Laid-Open No. H11-239729A,
Patent Document 13: Japanese Patent Laid-Open No. 2000-5604A,
Patent Document 14: Japanese Patent Laid-Open No. H03-262540A,
Patent Document 15: Japanese Patent Laid-Open No. H 11-508901A,
Patent Document 16: Japanese Patent Laid-Open No. H06-179640A,
Patent Document 17: Japanese Patent Laid-Open No. H07-2740A,
Patent Document 18: Japanese Patent Laid-Open No. S61-254256A,
Patent Document 19: Japanese Patent Laid-Open No. 2000-117114A,
Patent Document 20: U.S. Pat. No. 4,440,871 specification,
Patent Document 21: U.S. Pat. No. 5,126,308 specification,
Non-Patent Document 1: J. Liang, H. Li, S. Zhao, W. Guo, R. Wang, and M. Ying, Appl. Catal., 1991, 64, pp. 31 to 40,
Non-Patent Document 2: A. M. Prakash, S. Unnikrishnan, J. Chem. Soc. Faraday Trans., 1994, 90 (15), pp. 2291 to 2296,
Non-Patent Document 3: S. Ashtekar, S. V. V. Chilukuri, D. K. Chakrabarty, J. Phys. Chem., 1994, 98, pp. 4878 to 4883,
Non-Patent Document 4: D. R. Corbin, S. Schwarz, and G. C. Sonnichsen, Catalysis Today, 1997, 37, pp. 71 to 102,
Non-Patent Document 5: E. M. Flanigen, B. M. Lok, R. L. Patton, and S. T. Wilson, New Developments in Zeolite Science and Technology, Elsevier, 1986, pp. 103 to 112, and
Non-Patent Document 6: Structure Commission of the International Zeolite Association, Atlas of Zeolite Framework Types, Elsevier, 2001, pp. 14 to 15.
As described above, the present inventors have found that, among the SAPOs, an 8-membered ring SAPO of chabazite structure having a rectangular parallelepiped or cubic crystal form of 5 μm or less in average grain diameter works as an excellent methylamine production catalyst high in activity and dimethylamine selectivity without any modification after synthesis, and have already applied for patent (Japanese Patent Laid-Open No. 2000-117114A). However, it has been observed that the SAPO having the above grain diameter and shape still shows variation of catalyst performance depending upon production lots.
The above-described 8-membered ring SAPO of chabazite structure is obtained by hydrothermally treating a starting mixture that contains organic amines or organic ammonium salts such as tetraethyl ammonium hydroxide (TEAOH) and diethanolamine in an amount of 1.5±0.5 times the mole number of aluminum compounds as Al2O3. However, such a method as described above, in which an excessive amount of organic amines or ammonium salts is required, produces the catalyst at a low yield and needs treatment of the surplus organic amines or ammonium salts by activated sludge or the like.
Therefore, the SAPO disclosed in Japanese Patent Laid-Open No. 2000-117114A involves problems that must be further improved concerning catalytic stability for amine synthesis, the excess amount of structure directing agents, or the like.
Objects of the present invention are to solve the above problems and provide an 8-membered ring SAPO having excellent catalytic activity and dimethylamine selectivity, a method for stably producing the SAPO at a low cost, and a method for producing methylamines in the presence of the SAPO as a catalyst.